This invention relates to heat recovery steam generators (HRSG), and more particular to a HRSG with an ammonia injection system.
Natural gas and to a lesser extent fuel oil are the sources of much of the electrical energy consumed today. Combined cycle power plants convert those fuels into electrical energy in a highly efficient manner. There are three major components in a combined cycle power plant: a combustion turbine/electrical generator, a Heat Recovery Steam Generator (HRSG), and a steam turbine/electrical generator. Basically, the fuel, whether it is natural gas or oil, burns within the combustion turbine, and the turbine drives an electrical generator that produces some of the electrical energy furnished by the plant. The combustion turbine also discharges exhaust gas at elevated temperatures often exceeding 1000° F. The exhaust gas flows through the HRSG which extracts heat from it to convert subcooled water into superheated steam that flows into the steam turbine, which in turn drives another electrical generator that produces more electricity.
The typical HRSG has a casing into which the exhaust gas from the combustion turbine discharges and within the casing a succession of heat exchanges for extracting heat from the exhaust gas and transferring it to water flowing through the heat exchanges in various phases: subcooled, saturated, and superheated. The heat exchangers take the form of coil sections. Many HRSGs have a selective catalytic reduction (SCR) system located in the casing between two of the heat exchangers where the temperature of the exhaust gas is best suited for its operation. The SCR system controls the oxides of nitrogen (NOx) emissions emitted to atmosphere from the HRSG stack at the downstream end of the casing. For the NOx catalyst to function, ammonia must be injected into the exhaust stream. The ammonia and NOx are converted in the presence of the catalyst to nitrogen and water vapor. Ammonia is generally delivered to plant sites diluted with water for safer handling. The diluted ammonia is called aqueous ammonia. Prior to injection into the HRSG, the aqueous ammonia is vaporized by mixing it with a hot carrier gas. The carrier gas also provides dilution of the ammonia, and it is then introduced into the exhaust gas flowing through HRSG through a series of distributors. The carrier gas can be electrically heated air or it can be extracted with a high temperature combustion turbine exhaust gas. When the combustion turbine exhaust gas is used, it is extracted from the HRSG very close to and upstream of the point of reinjection at the distributor so as to reduce overall heat loss from the HRSG and, thus, not penalize the HRSG steam production efficiency. This extracted exhaust gas temperature must be above ˜450° F. before the aqueous ammonia flow can be established.
Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.